422
9 AUTOCHTHONOUS SEDIMENTS
occur in sabkha facies, interbedded with fecal pellet muds, stromatolitic algal limestones, and evaporites. Associated sedimentary structures include desiccation cracks
and "tepee" structures (the wigwam-like buckling of bedding due to penecontemporaneous hydration and expansion of anhydrite; see, for example, Kendall, 1969).
9.2.6.3 Secondary Dolomites
Secondary dolomites are defined as those that are obviously of postdepositional origin. This is clearly shown by the way in which such dolomites have an irregular distribution, discordant to bedding and cross-cutting sedimentary structures. Unlike primary
dolomite this type has crystals of more than 20/xm in diameter, which are occasionally
euhedral or idiomorphic and cross-cut relic microfabrics of the original limestone
(Plate 1C). These secondary dolomites have a characteristic sugary texture (sometimes
referred to as sucrosic or saccharoidal). This has resulted in part from the bulk volume
shrinkage (as calcite is replaced by dolomite) and in part from the dissolution of residual calcite during the final stages of dolomitization. Thus secondary dolomites are frequently porous with intercrystalline pores connected to one another by planar throat
passages. Unlike primary dolomites these secondary dolomites can act as excellent hydrocarbon reservoirs.
Several models for secondary dolomitization have been proposed, and there are supporters and critics of each. Most of the models are based on the idea that dolomitization
takes place when brines of high Mg:Ca ratio flow through permeable limestone. The
reaction is a straightforward replacement according to the formula:
2CAC03 + Mg 2+ ~ CaMg(C03) + Ca 2+.
Note that this is a reversible reaction -- dedolomitization (or calcitization) is also known
to occur (Shearman et al., 1961). One of the most popular models for secondary dolomitization by magnesium enriched brine is the "seepage reflux" or "leaky dam" mechanism. Consider a lagoon or restricted embayment in an arid climate. Seawater flows into
the lagoon. Calcium ions are removed from the lagoonal water, both by the secretion
of lime by organisms and, as salinity increases, by the precipitation of gypsum. Thus the
Mg: Ca ratio increases. Continued evaporation may increase the density of the water
until it begins to flow downward and seaward through the permeable reef or carbonate
sand barrier. The magnesium-enriched brines dolomitize the limestones as they pass
through them (Fig. 9.15). The Bonaire Lagoon in the Antilles has been cited as a modern example of this process (Deffayes et al., 1965), and the Stettler Formation of the Williston basin, Canada, cited later as an ancient analog (see Section 9.6.3). Nonetheless,
as with every proposed model of dolomite formation, the "leaky dam" scenario has its
critics (e.g., Wells, 1986; Budd, 1997).
The one fact that should not be lost sight of in all the foregoing is that primary dolomites tend to be microcrystalline and porous, but impermeable. Secondary dolomites,
on the other hand, tend to be coarsely crystalline, with good intercrystalline porosity.
Because of their coarse crystals they have large pores and pore throats, so they may be
highly permeable. Primary dolomites do not make good petroleum reservoirs, without
leaching and/or fracturing, whereas secondary dolomites do make excellent reservoirs.
9 AUTOCHTHONOUS SEDIMENTS
occur in sabkha facies, interbedded with fecal pellet muds, stromatolitic algal limestones, and evaporites. Associated sedimentary structures include desiccation cracks
and "tepee" structures (the wigwam-like buckling of bedding due to penecontemporaneous hydration and expansion of anhydrite; see, for example, Kendall, 1969).
9.2.6.3 Secondary Dolomites
Secondary dolomites are defined as those that are obviously of postdepositional origin. This is clearly shown by the way in which such dolomites have an irregular distribution, discordant to bedding and cross-cutting sedimentary structures. Unlike primary
dolomite this type has crystals of more than 20/xm in diameter, which are occasionally
euhedral or idiomorphic and cross-cut relic microfabrics of the original limestone
(Plate 1C). These secondary dolomites have a characteristic sugary texture (sometimes
referred to as sucrosic or saccharoidal). This has resulted in part from the bulk volume
shrinkage (as calcite is replaced by dolomite) and in part from the dissolution of residual calcite during the final stages of dolomitization. Thus secondary dolomites are frequently porous with intercrystalline pores connected to one another by planar throat
passages. Unlike primary dolomites these secondary dolomites can act as excellent hydrocarbon reservoirs.
Several models for secondary dolomitization have been proposed, and there are supporters and critics of each. Most of the models are based on the idea that dolomitization
takes place when brines of high Mg:Ca ratio flow through permeable limestone. The
reaction is a straightforward replacement according to the formula:
2CAC03 + Mg 2+ ~ CaMg(C03) + Ca 2+.
Note that this is a reversible reaction -- dedolomitization (or calcitization) is also known
to occur (Shearman et al., 1961). One of the most popular models for secondary dolomitization by magnesium enriched brine is the "seepage reflux" or "leaky dam" mechanism. Consider a lagoon or restricted embayment in an arid climate. Seawater flows into
the lagoon. Calcium ions are removed from the lagoonal water, both by the secretion
of lime by organisms and, as salinity increases, by the precipitation of gypsum. Thus the
Mg: Ca ratio increases. Continued evaporation may increase the density of the water
until it begins to flow downward and seaward through the permeable reef or carbonate
sand barrier. The magnesium-enriched brines dolomitize the limestones as they pass
through them (Fig. 9.15). The Bonaire Lagoon in the Antilles has been cited as a modern example of this process (Deffayes et al., 1965), and the Stettler Formation of the Williston basin, Canada, cited later as an ancient analog (see Section 9.6.3). Nonetheless,
as with every proposed model of dolomite formation, the "leaky dam" scenario has its
critics (e.g., Wells, 1986; Budd, 1997).
The one fact that should not be lost sight of in all the foregoing is that primary dolomites tend to be microcrystalline and porous, but impermeable. Secondary dolomites,
on the other hand, tend to be coarsely crystalline, with good intercrystalline porosity.
Because of their coarse crystals they have large pores and pore throats, so they may be
highly permeable. Primary dolomites do not make good petroleum reservoirs, without
leaching and/or fracturing, whereas secondary dolomites do make excellent reservoirs.